Prosthetic hip joint devices, such as the P.C.A. hip system distributed by Howmedica, Inc. and the APR Universal System distributed by Intermedics Orthopaedics, are known and in wide use in current medical practice.
The insertion and attachment of prosthetic devices may be either cemented, using bone cement, or through ingrowth of surrounding bone into the prosthesis/bone interface. More recently, the use of uncemented joint replacements has been favored as surgeons wish to avoid the use of bone cement and its attendant difficulties and potential complications. In order to achieve appropriate bone ingrowth, porous coatings are used on various portions of the prosthetic device. Porous metal coatings for surgical prosthetic devices are described in U.S. Pat. No. 3,855,638 to Pillar. The porous coated structure encourages bone ingrowth and enhances cement fixation; however, fixation is preferably achieved biologically with bone ingrowth.
A porous coating in itself will not ensure that the surgeon's goal, i.e., long-term fixation, is realized in order to avoid early failure and subsequent revision. Existing porous-coated systems do not properly address this problem because: (1) prior total hip systems are not stabilized initially because of problems such as micro movement, subsidence and torsional loosening and, (2) imprecise coating of the prosthesis can cause a phenomena known as "stress shielding" and loosening which, when they occur, can lead to catastrophic failure in a relatively short time after implantation.
Also, prior femoral stem implants limit the area of porous coating to the calcar region; the implant is usually set in soft cancellous bone. With many prior femoral stem implants, clinicians have reported a significant incidence of subsidence or sinking of the prosthesis stem portion into the femur. Any subsidence is usually viewed as a failure, since it can cause severe patient pain and result in loosening and eventual replacement or the prosthesis. In many cases fibrous tissue attachment, not true bone ingrowth, results and thus may contribute to implant loosening and failure.
The total hip system of the present invention provides a combination of the correct prosthetic design to stabilize the hip immediately upon implantation and specific placement of the porous coating, both carefully selected to overcome the risk of loosening and stress shielding.
The total hip system described herein includes three essential components. They are (1) a femoral stem, anatomically shaped and easily constructed in right and left configurations and a range of sizes, (2) a femoral head which attaches to the stem intraoperatively, and (3) an acetabular component consisting of a polymer bearing-type insert mechanically fixed inside a metal shell, the bearing surface articulating with the femoral head.
An object of the present invention is to provide a porous coated total hip replacement system having initial stability so that bone in-growth can occur and a porous metal coating in predetermined regions with an appropriate structure that encourages bone ingrowth and/or enhances cement fixation to the required location. An anatomically designed femoral stem with a collar which is coated on the underside for added cortical fixation, physiological stress transfer and prevention of subsidence and a cooperating acetabular component to provide the optimum success rate for implanted devices are combined with a porous ingrowth area strategically located in the proximal femoral region to result in not only ingrowth of the soft cancellous bone, but also adequate ingrowth of hard, true cortical bone tissue. The acetabular component is provided with both a porous coating and also means for mechanical attachment and fixation to the pelvis via screws or other fastening devices.
A porous metal coating is located in the upper proximal half of the femoral stem and is selected to be applied at a suitable thickness of an optimal pore size and with the appropriate porosity to ensure compatibility with and ingrowth by the surrounding bone tissue. The femoral stem component of the total hip replacement system is designed to anatomically fill the femoral canal, is provided in both left and right hand configurations, and causes minimal bone stock removal. The anatomically designed femoral stem with collar serves to resist torsional loosening by means of its curvature and rectangular/triangular anterior/posterior cross section to stress the calcar physiologically, provide a calcar cortical in-growth interface and to reduce subsidence of the stem into the bone which, together with the placement of the porous coating extending into the femoral cortical or hard bone, serves to further prevent subsidence of the stem. These multiple features are important and must be used in combination in order to achieve best results, for a collarless anatomical stem even when provided with the porous coating can contribute to torsional loosening if it has a rounded cross-section and if the porous coating is limited to the soft cancellous bone. Thus, the stem component of the porous coated total hip replacement of the present invention provides a curvature with rectangular/triangular anterior/posterior cross-section and a porous coating that extends into the proximal cortical area of the femur and thereby effectively resists torsional loosening.
The design of the femoral stem in accordance with the invention is also selected to minimize or avoid subsidence or sinking of the implant into the femur while maintaining fixation strength that can only be achieved with an implant that is fixed both to the cancellous and the cortical bone and in combination with a medial collar reinforced for initial stability of the implant with porous coating on the underside to provide added cortical ingrowth, physiological stress transfer and to further prevent subsidence. Fixation to the cortical bone is a particular advantage since it serves to transfer stress from the prosthesis to the adjacent cortical bone as well as to adjacent cancellous bone. Absence of successful stress transfer, for instance fixation only to the cancellous area, will invite difficulties such as stress shielding, subsequent loosening and subsidence. Although adapted to be used either with or without bone cement, the porous coated total hip replacement system of the present invention may be successfully used as an uncemented joint replacement and a stable bone/prosthesis interface results.
The total hip replacement system as described herein is intended for reconstruction of painful and/or severely disabled hip joints resulting from osteoarthritis, rheumatoid arthritis, traumatic arthritis, avascular necrosis and other conditions, or as a replacement for previously failed prostheses. The device according to this invention employs an anatomically shaped stem whose curvature closely follows the proximal portion of the femur thus minimizing bone removal while providing for uniform stress transfer. The undercoated collar enhances stability of proximal fixation, physiological stress transfer proximally and provides security against subsidence. Pore size, thickness and placement of the porous coating provide optimum long term fixation and stress distribution while providing a stable interface for bone ingrowth.
The total hip replacement system has unique advantages which, in the hands of the orthopaedic surgeon, restores the patient to naturally functioning, pain-free mobility. The unique features of the system are the initial stability of the implant which allows in-growth to occur, the specific porous coated parameters of pore size, pore volume and the specific placement of the porous coating interface on the femoral stem, the femoral stem collar and acetabular cup to facilitate long term stability and vacuum casting of the femoral component to provide added strength in high cycle fatigue loading.